1. Field of the Invention
The present invention relates to an inkjet recording medium. More specifically, the present invention relates to an inkjet recording medium preferable for use with both dye and pigment inks.
2. Description of the Prior Art
Inkjet recording generally involves ejecting small droplets of ink using various mechanisms and forming dots by allowing the droplets to adhere to a recording medium. Inkjet recording is less noisy than dot impact recording, can readily provide full color prints, and offers the advantage of potential utility for high speed printing.
Ink jet recording processes are traditionally conducted using mainly aqueous dye inks. Such aqueous dye inks use low molecular weight dye compounds as coloring agents. Although these compounds develop color well, they also have problems. For example, they blur easily when exposed to water and the like, and the colors fade and change upon extended exposures to light and gases due to the structure of the coloring agents resulting in problems associated with preservative property of recorded images and image durability.
Therefore, inks formed using pigments as coloring agents were put into practice in order to overcome the problems associated with dye based inks and to improve waterfastness and lightfastness. (See, for example, Unexamined Japanese Patent Publications (Kokai) Hei 11-20306, 2000-79752 and 2003-145916.) However, when a pigment based ink is used to print on a conventional inkjet recording medium designed for dye based inks, problems occurred as optical (image) density declined and lack of solid image uniformity. Furthermore, when a larger amount of pigment based ink is ejected in order to promote better color development, the coloring agents accumulate on the recording medium surface resulting in lowered abrasion resistance, staining of printed materials and disruption of the ink solvent absorption due to the accumulation of coloring agents.
Therefore, dyes and pigments have recently been used simultaneously in inkjet recording inks, and a recording medium compatible with both dye based and pigment based inks is urgently needed. A technology to improve the recording property of both dye based and pigment based inks by adding a fine inorganic particles and an adhesive comprising a vinyl chloride-vinyl acetate copolymer to the ink absorbing layer has been disclosed. (See, for example, Unexamined Japanese Patent Publication (Kokai) 2001-270238.) However, this technology failed to yield a satisfactory printing property, particularly when ink absorption and optical density in printing using a pigment based ink are concerned.
Simultaneously, opportunities to output (print hard copy of) high resolution images using inkjet printers are increasing due to the popularity of high resolution digital video, digital cameras, scanners and personal computers. As a result, new demands are placed on inkjet recording media. That is, faster ink drying speed, high optical density, the absence of ink blurring and bleeding, and the absence of cockle upon ink absorption as well as providing gloss comparable to that of silver halide photographs are in demand.
In order to satisfy these properties, a technology to manufacture recording media using a cast coating method has been proposed. (See, for example, Unexamined Japanese Patent Publications (Kokai) Sho 62-95285, Hei 02-274587, Hei 05-59694, Hei 06-305237, Hei 09-156210 and Hei 11-48604.) The cast coating method proposed in these publications yields a high gloss cast coated paper by applying an ink receiving layer comprising a pigment, the major components of which are a synthetic silica, and a binder, pressing said layer onto a heated mirror finished surface while the layer is still wet to transfer the mirror finished surface and simultaneously dry it. However, the gloss of the outermost surface layer is still inadequate and a gloss, comparable to that of a silver halide photograph cannot be obtained-even using this technology. In addition, the recording property using a pigment ink is not good.
An addition of 5-50 nm spherical colloidal silica into the abovementioned cast layer is tried to obtain a high gloss, whrein the silica is dispersed in water to form a stable colloid that does not undergo secondary aggregation. (See Unexamined. Japanese Patent Publications (Kokai) Hei 05-338348 and Hei 10-217599.) This colloidal silica is composed of fine particles, and a very clear and high gloss coating film is obtained when it is dried. In addition, technologies in which said cast layer contains (1) fine silica particles having an average particle diameter for primary particles of 3 nm to 40 nm and an average particle diameter for secondary particles of 10 nm to 400 nm, and (2) colloidal silica having an average particle diameter of 200 nm or less have been reported. (See, for example, Unexamined Japanese Patent Publication (Kokai) 2000-85242.)
However, almost all colloidal silica consists of truly spherical particles, and primary particles are singly dispersed without aggregation. Therefore, the particles are tightly packed when dried, and very little inter-particulate gaps exist. As a result, the pore volume obtained using colloidal silica is generally low, under 0.4 ml/g. When this silica is added to a cast layer, the ink absorption rate is slowed and causes inks blurring and uneven image density.
In addition, a recording paper on which a glossy layer containing a pearl necklace (beaded) type colloidal silica and the like is applied over an ink absorbing layer without using a cast coating method has been proposed. (See. Unexamined Japanese Patent Publications (Kokai) 2000-108505, 2000-108506 and 2000-62314.) Furthermore, a technology in which an ink absorbing layer is constructed from more than one layer and at least one of the layers contains a cationic resin and colloidal particles having an average particle diameter of 300 nm or less has been reported. (See, for example, Unexamined Japanese Patent Publication (Kokai)-Hei 09263039.)
This technology is good for developing color and absorbing ink when with a dye ink. However, ink particles do not anchor well into a glossy layer and images break apart when they are touched and images stain other white paper section when a pigment ink containing coloring particles having a particle diameter of from 50 nm to 150 nm is used.
Alternatively, fine synthetic silica particles formed using a vapor phase method were added to an ink absorption layer. (See Unexamined Japanese Patent Publications (Kokai) Hei 10-81064 and Hei 11-34481.) Silica formed using a vapor phase method is composed of super fine particles, the average particle diameter of primary particles is from several nanometers to several tens of nm, have excellent dispersion properties, have excellent transparency, are bulky and are more readily converted into aqueous dispersions than silica formed using a wet method. A high gloss coating film having good ink absorption properties can be formed when such an aqueous dispersion is, coated. Silica formed using a vapor phase method can be manufactured by exposing a volatile silicon compound to a flame to induce decomposition at high temperatures. (See, for example, Unexamined Japanese Patent Publication (Kokai) Sho 59-169922.)
However, the inter-particulate bonding of aggregated particles of silica formed using a vapor phase method is relatively weak, and the aggregated state is disrupted by the capillary force generated by the voids created when water is dried to form a coating film. The cast layer tends to form fine, turtle shell-like cracks that may be observed by optical microscope.
As described here, the abovementioned problems are encountered when a colloidal silica having a small particle diameter or a silica formed using a vapor phase method is used to achieve a high gloss.
In addition, a so-called uneven printing is sometimes encountered, particularly in cyan-colored printings, as recording media become glossier. Printing non-uniformity refers here to uneven image density when a solid image is printed using an inkjet recording method.